Conventional processes for the production of triorganotin monohalides usually involve reacting the corresponding tetraorganotin compound with stannic chloride; the tetraorganotin being initially made via a Grignard process or an aluminum alkyl process.
However, these are inefficient, multistage, indirect, processes, and various investigators have searched for direct processes. British patent specifications Nos. 1,432,047 and 1,450,490 describe processes which attempt to control the Grignard process and stop the reaction at the stage when three carbon-to-tin bonds are formed. Such control is very difficult to achieve. U.S. Pat. Nos. 3,471,539 and 3,475,472 describe processes for adding a third carbon-tin bond to a pre-formed diorganotin compound using another metal (e.g., zinc), i.e., EQU R.sub.2 SnX.sub.2 +RX+Zn.fwdarw.R.sub.3 SnX+ZnX.sub.2.
Another U.S. Pat. No. 3,547,965 describes a process for the direct preparation of triorganotin halides using a stoichiometric amount of zinc, i.e., EQU 3RX+Sn+Zn.fwdarw.R.sub.3 SnX+ZnX.sub.2.
Still another U.S. Pat. No. 2,852,543 describes a process using two additional metals (sodium and zinc) which can produce a mixture of tri- and tetraorganotins and which is mainly triorganotin.
All of these processes involve another metal, as well as tin, and therefore do not offer any real advantage over the traditional multistage processes.
As mentioned earlier, tetraorganotin compounds are commonly made via a Grignard process, or an aluminum alkyl process. Less common processes use molten sodium. In all of these cases the tin is usually present as the tetrahalide, e.g., the tetrachloride. However, direct processes to make tetraorganotins from elemental tin have also been described.
Thus U.S. Pat. No. 3,651,108 describes the preparation of tetraorganotin compounds by reaction of organic halides in the presence of an 'onium compound, or Lewis base, with tin and an alkali or alkaline earth metal, in particular magnesium. Similarly U.S. Pat. Nos. 4,179,458 and 4,092,340 describe processes for preparing tetraorganotin compounds which comprise reacting an organic halide of formula RX with a heated suspension of metallic material which is zinc and tin (in the atomic preparations of at least 0.5 to 1) in a liquid comprising at least one 'onium salt, which salt is an organic quaternary ammonium or phosphonium or tertiary sulphonium salt, to produce the tetraorganotin compound. In U.S. Pat. No. 4,179,458 this agent is a liquid, in U.S. Pat. No. 4,092,340, it is a gas. From the examples given in these two cases, the processes appear to be EQU 4RX+Sn+2Zn.fwdarw.R.sub.4 Sn+2ZnX.sub.2.
The liquid 'onium compound functions as a solvent.
Both specifications state that the tetraorganotin product may be used for preparing triorganotin halo compounds by mixing with the appropriate molar proportion of stannic halide, e.g., stannic chloride.
Direct processes (i.e., involving tin only as the metal) for the production of diorganotin compounds are well known. For example see a review article by Murphy & Poller, The Preparation of Organotin Compounds by the Direct Reaction, J. Organomet. Chem. Lib., (1979), 9, 189-222.
The direct reaction of tin with an organic halide in the presence of catalytic amounts of a quaternary ammonium and phosphonium compound or of a ternary sulphonium or isosulphonium compound to produce primarily mono- and diorganotin halides (possibly with triorganotin halides as a minor product) is disclosed in several earlier patent specifications, for example, British specifications Nos. 1,115,646, 1,053,996 and 1,222,642, the respective disclosures of which are incorporated herein by reference.
Specification No. 1,115,646 discloses reacting metallic tin (which may be used in powder, sheet or granule form, and may be part of an alloy, especially with a co-catalyst) with an aliphatic halide in the presence of a catalyst, which is an 'onium compound (defined as a compound containing organic groups covalently bonded to a positively charged non-metallic atom from Group V or VI of the periodic table, and exemplified by tetraalkylammonium halides, tetraalkyl phosphonium halides and trialkylsulphonium halides) and in the presence of a preformed stannous halide or pre-formed organotin halide, and optionally in the presence of a small amount of a co-catalyst which is one of thirteen defined metals (said small amount being up to 0.1 mole per gram-atom of tin). The product of the reaction of Specification No. 1,115,646 is a mixture of organotin halides in which the diorganotin dihalide predominates.